1. Field of the Invention
The present invention pertains to an insufflation system and method that delivers a flow of insufflation gas to the airway of a patient to remove expired gases from a patient's anatomical dead space and/or the structural dead space in a breathing circuit during ventilation, and, in particular, to an insufflation system and method that delivers a flow of insufflation gas to the patient's airway in such a manner so as to minimize stagnation pressure in the patient's lungs due to the flow of insufflation gas into the patient and to an insufflation system and method that can be used in conjunction with a conventional ventilation system without altering the operation of the conventional ventilation system.
2. Description of the Related Art
It is known to reduce rebreathing of exhaled gases in an intubated patient or in a patient with a tracheostomy by providing a flow of insufflation gas, such as oxygen, an oxygen mixture, or other therapeutic gas, into the distal end of the patient's breathing circuit. FIG. 1 illustrates an example of such a conventional system, commonly referred to as a tracheal gas insufflation (TGI) system, in which a flow of insufflation gas is delivered to the airway of the patient. A primary flow of breathing gas that augments or completely supports the patient's breathing is delivered using a conventional ventilator.
As shown in FIG. 1, an endotracheal tube 30 inserted into an airway 32 of a patient 34 through the oral cavity delivers the primary flow of breathing gas from a ventilator 36 to the patient's lungs 38. In such a conventional ventilation system, a breathing circuit 40 delivers the primary flow of breathing gas from the ventilator to the patient via a first limb 42, and exhaled gas from the patient is removed via a second limb 44. First and second limbs 42 and 44 are typically flexible tubes coupled to endotracheal tube 30 via a coupling member, such as a Y-adapter. For purposes of this invention, the breathing circuit includes all of the structures associated with the ventilation system that communicate the primary flow of breathing gas with the airway of the patient, such as first limb 42, second limb 44, endotracheal tube 30 and any coupling members.
As the patient inspires, the primary flow of breathing gas is delivered by ventilator 36 to the patient's respiratory system, i.e., the airway and lungs, via breathing circuit 40. Typically, the primary flow of gas delivered to the patient by the ventilator is controlled based on the total volume delivered, the pressure of the gas delivered, or the patient's respiratory effort, the latter of which is known as proportional assist ventilation (PAV). While an endotracheal tube, which is passed into the patient's airway via the oral cavity, is illustrated in FIG. 1 as being part of the breathing circuit, it is to be understood that other methods for delivering and/or interfacing breathing gas to the patient, such as a tracheostomy tube, nasal and/or oral mask, or an nasal intubated endotracheal tube, are commonly used in conventional ventilation systems as part of the breathing circuit.
As the patient expires, i.e., breathes out, the exhaled gas, which is laden with CO2, is removed from the lungs and airway via endotracheal tube 30 and second limb 44 of breathing circuit 40. Typically, an exhaust valve (not shown) associated with second limb 44 and operating under the control of ventilator 36 manages the flow of exhaust gas from the patient so that, if desired, a certain level of positive end-expiratory pressure (PEEP) can be maintained in the patient's respiratory system. In some ventilation systems, the second limb includes an exhaust valve that is controlled by the ventilator but is not contained within the ventilator itself.
It can be appreciated that at the end of exhalation, not all of the exhaled gas containing CO2, for example, is exhausted to atmosphere. A certain amount of exhaled gas remains in the physiological and anatomical dead space within the patient and in the structural dead space within the breathing circuit. The structural dead space in the breathing circuit is the portion of the breathing circuit beginning at a distal end 55 of endotracheal tube 30 or tracheostomy tube to a location 46, where the exhalation (second) limb 44 separates from the rest of the breathing circuit. It is generally desirable to prevent the exhaled, CO2 laden gas in this dead space from being rebreathed by the patient, so that the patient receives the maximum amount of oxygen or other therapeutic gas and a minimal amount of CO2 during each breath. In some patients, such as patients with cranial injuries, it is imperative that their CO2 level not be elevated.
Tracheal gas insufflation (TGI) is one method that attempts to remove the exhaled gas from the physiological, anatomical and structural dead spaces in a patient being treated with a ventilator. Tracheal gas insufflation involves introducing an insufflation gas, such as oxygen, an oxygen mixture, or other therapeutic gas, into the patient's airway 32 at the distal end of breathing circuit 40. In the embodiment illustrated in FIG. 1, an insufflation gas source 48, such as a pressurized tank or oxygen or an oxygen wall supply, delivers a flow of insufflation gas via a conduit 50 as a stream of gas into the patient's airway. Conduit 50 is also referred to as an “insufflation catheter.” In a conventional TGI system, a proximal end of conduit 50 is coupled to insufflation gas source 48 through a control valve 52, and a distal end of conduit 50 is located generally within or near the distal end of endotracheal tube 30 so that the flow of insufflation gas is directed toward lungs 38, as indicated by arrow 54. Typically, the distal end of conduit 50 is located just above the patient's carina. The oxygen rich flow of insufflation gas discharged from the distal end of conduit 50 displaces the exhaled air in the anatomical and structural dead spaces so that the patient inhales the fresh (non CO2 laden) gas on the next breath, thereby minimizing rebreathing of CO2 to keep the patient's CO2 levels as low a possible.
Conventionally, there are two techniques for delivering the flow of tracheal insufflation gas to a patient. According to a first TGI technique, the flow of insufflation gas is delivered to the patient continuously during the entire breathing cycle while the ventilator delivers the primary flow of breathing gas to the patient. This technique is commonly referred to as a “continuous TGI system.” This continuous TGI delivery method, however, has a significant drawback in that conventional ventilators are not capable of accounting for the additional volume of gas delivered to the patient by the continuous TGI system. As a result, the extra volume of gas bled into the breathing circuit by the continuous TGI system is simply summed with the prescribed volume of gas being delivered by the ventilator. A possible outcome is that an excessive pressure of gas is delivered to the patient, possibly over-inflating the patient's lungs. This excessive pressure is referred to as “autoPEEP.” A disadvantage associated with autoPEEP is that it increases the patient's work of breathing, because in order to initiate inspiration, the patient must generate an inhalation force that is strong enough to overcome the autoPEEP pressure. AutoPEEP may also cause tissue damage due to the hyper-inflation of the patient's lungs.
These problems are dealt with, at least in part, in conventional continuous TGI systems by carefully adjusting the ventilator settings to avoid over-inflation. It can be appreciated that “fooling” the ventilator so that the continuous flow of insufflation gas does not over-inflate the patient's respiratory system is not an ideal solution because it does not maximize the operating abilities of the ventilator. The ventilator must be specifically configured to deal with this extra insufflation gas, rather than being configured as it normally would in the absence of the flow of insufflation gas. On the other hand, maximizing the operating characteristics of the ventilator by setting it up without accounting for the flow of insufflation gas may result in excessive CO2 levels in the patient or hyperinflation of the patient. In addition, adjusting the operating characteristics of the ventilator to prevent over-inflation when a continuous TGI system is used requires a highly trained operator to make the correct fine-tuning adjustments to the ventilator. Furthermore, this continuous TGI technique requires constant monitoring of the patient and ventilator system because changes in the patient's breathing cycle that may require reconfiguring of the ventilator or the continuous TGI system can occur in very short time periods.
According to a second TGI technique, referred to as a “phasic TGI system,” the flow of insufflation gas is controlled so that the insufflation flow is only delivered to the patient during the expiratory phase, preferably at the end, while the exhaust valve associated with the second limb of the breathing circuit is open. Because the exhaust valve is open when the flow of insufflation gas is delivered, the extra volume of insufflation gas being delivered to the patient displaces an equal volume of gas out of the breathing circuit through the exhaust port and, therefore, does not over-inflate the patient's lungs. This phasic approach, however, requires a relatively complicated control mechanism for controlling the flow of insufflation gas in conduit 50, for example, by controlling valve 52 using ventilator 36, to ensure that the flow of insufflation gas is only delivered while the exhaust valve associated with second limb 44 is open. It can be appreciated that this phasic TGI technique increases the complexity and cost of the ventilation system and the TGI system due to the precise timing required to control the operation of the ventilator and valve 52, so that the gas is delivered at the correct time during the patient's breathing cycle.
Another drawback associated with conventional TGI systems, including both the continuous and phasic TGI techniques, is that autoPEEP is also caused by a phenomenon known as stagnation pressure. Stagnation pressure, also known as dynamic pressure, is the pressure or force generated when a flowing gas is brought to rest by isentropic flow against a pressure gradient. The magnitude of the stagnation pressure is proportional to the square of the change in velocity of the gas. Because the insufflation gas in a conventional TGI system is directed into the patient's airway using a relatively small diameter tubing, typically 0.1 inch diameter, it has a relatively high velocity, which is decelerated into a closed volume, namely the patient's airway and lungs. As a result, a stagnation pressure is created within the patient, thereby exacerbating the autoPEEP problem. It should be noted that the problem of autoPEEP due to stagnation pressure is prevalent in both the continuous and phasic TGI systems because the timing at which the flow of insufflation gas is introduced into the patient does not affect the magnitude of the stagnation pressure generated.